It is generally accepted that escalating concentrations of atmospheric carbon dioxide (CO2) are driving changes in climate patterns. Policy mechanisms such as ‘Reducing Emissions from Deforestation and forest Degradation’ (or REDD+) aim to reduce CO2 levels in the atmosphere through compensating landowners to manage their land as carbon sinks. However, for such a scheme to succeed accurate quantification and reporting of the sequestered carbon must be conducted using verifiable methodology. Vegetated coastal habitats, such as mangrove forests, provide an opportunity to develop a carbon offset project.

In Kenya, mangroves face a myriad of human and natural induced stresses ranging from over-exploitation of resources, conversion pressure, and sea level rise. The degradation presents an opportunity for engaging in carbon markets through rehabilitation, conservation and sustainable utilization of mangrove resources. This study at Mwache creek, in Mombasa, aimed at estimating total mangrove carbon stocks in the area; in order to provide baseline information in which future offset projects could be based. Systematic stratified sampling technique was used in the study. Three carbon pools were considered, viz: Above ground, below ground (root) and soil carbon pools. Soil cores were collected at the center of 10 x 10 m2 plots laid 100 m apart along transects. For each soil core, four sub-samples; viz., 0-15; 15-30; 30-50; and 50-100 cm were extracted for analysis of soil structure, bulk density and carbon concentration. Wet sieving was used to determine soil structure; whereas organic matter and carbon concentration were determined using loss on ignition (LOI) and the colorimetric methods. The study results indicate a statistical difference (p<0.05) in the vertical distribution of soil organic carbon but no statistical difference (p>0.05) in the horizontal distribution along the sea-land transects. A statistical difference (p<0.05) in the soil carbon was observed across degradation gradients with less degraded sites exhibiting higher concentrations. Above and below ground biomass was obtained using published allometric equations (230.6 and 82.7 Mg ha-1, respectively) and used to determine associated carbon. The derived above and below ground carbon was added to the soil carbon to obtain total mangrove carbon of the area. The total mangrove carbon in Mwache was estimated at 388.92 Mg C ha-1 of which 63% was soil carbon, 28% above ground carbon, and 9% below ground carbon. These findings provide a good baseline data for establishment of a small scale blue carbon project in the area.

Background information
A continuous cycle of carbon between earth, atmosphere and ocean exists. There is evidence that man has largely influenced this cycle leading to increased carbon dioxide (CO2) concentration into the atmosphere; and hence climate change (IPCC, 2007). It is estimated that tropical deforestation contributes approximately 18% emission of greenhouse gases (GHGs) into the atmosphere (IPCC, 2007); much of which is CO2. For this reason, the role of forests in mitigating climate change effects is recognised by the Land Use and Land Use Change and Forestry (LULUC-F) sector of United Nations Framework Convention on Climate Change (UNFCCC) (Brown et al., 1999); as forests sequester CO2 during the process of photosynthesis.

Carbon emission avoidance practices are encouraged to conserve existing carbon pools in forest vegetation and soil through options such as controlling deforestation or logging and other anthropogenic disturbances. A set of policies known as ‘Reducing emissions from avoided deforestation and forest degradation’ or REDD+ were introduced during the 11th session of the UNFCCC, in December 2005, and won support from almost all Parties, intergovernmental organizations and non-governmental organizations. REDD+ is concerned with both reducing emissions and enhancing carbon stocks through actions that address deforestation, forest degradation, forest conservation and sustainable forest management. The basic idea behind REDD+ is that countries that are willing and able to reduce emissions from deforestation and forest degradation should be compensated for doing so (Angelsen, 2008).

A key challenge for successfully implementing any REDD+ project is the reliable estimation of biomass carbon stocks in forests. Lack of information and inaccurate quantification of total sequestered carbon has made it difficult to establish the potential value of the ecosystems in global estimates and in trading of carbon credits in carbon financing programs such as REDD+. The deficiency is worse in mangrove forests owing to the logistic difficulties of working in the wetland ecosystem (Tamooh et al., 2008). While several studies have been published on above ground carbon stocks in the forests around the world, there is quite limited data on below ground carbon and particularly the soil carbon (Dargusch et al., 2010; Kauffman and Donato, 2012). Quantification of carbon storage in the mangroves has primarily been based on extrapolation from only a few forest surveys and inventory data (Komiyama et al., 2008). The present study aimed to complement global initiatives of determining carbon stocks of coastal wetlands, commonly referred to as “Blue Carbon”. The study focused on mangrove forests with an aim to provide baseline data for future engagement in carbon offset projects.

Statement of the problem
Considering the threats posed by climate change, particular interest needs to be given to cheaper ways of removing excess CO2 from the atmosphere. Despite occupying around 2% of the seabed area, vegetated ecosystems including mangroves, seagrass beds and salt marshes transfer 50% of carbon from the ocean to sediments which mostly build up continuously while storing the carbon (Crooks et al., 2010). However, these ecosystem are threatened by both human and natural induced stresses including, overexploitation of resources, conversion pressure and sea-level rise. Between 1980 and 2000 for instance, 35% of mangroves were lost globally (Giri et al., 2011). In Kenya, losses of mangroves from 1985 to 2010 has been estimated at 18% (Kirui et al., 2013); with peri-urban systems of Mombasa recording up to 86% cover loss (Olagoke, 2012; Bosire et al.,2013). Degradation of mangroves leads to loss of ecosystem services; and discharge of previously buried carbon from the mangrove ecosystems.

Despite the potential role of mangroves as carbon sinks large uncertainties exist regarding the amount of carbon stored in the forests and particularly in their soils. Further, their variability in relation to their positioning- fringing, riverine or estuarine, basin, over-wash islands or dwarf mangroves- brings about variations in their capacity to capture and store carbon consequently leading to difficulties for a general approach in quantification. This hence calls for site-specific studies of the carbon stocks and sequestration, which would matter greatly in forest conservation and in the issues of spatial and temporal change. This study was thus undertaken to accurately quantify the Mwache Creek mangrove forest carbon stocks as a precursor for a carbon offset project for the area.

Broad objective
To assess the total organic carbon in mangroves of Mwache Creek, Mombasa; in order to provide baseline data for future engagement in carbon offset projects.

Specific objectives
i. To determine horizontal and vertical distribution of soil organic carbon along sea-land transects in Mwache Creek.

ii. To correlate soil organic carbon with mangrove degradation gradient in Mwache Creek.

iii. To use the data to estimate ecosystem carbon stocks in Mwache Creek.

H01 There is no change in levels of soil organic carbon along the sea-land transects in Mwache Creek.

H02 There is no change in the quantity of soil organic carbon with an increase in depth in Mwache Creek.

H03 There is no variation in the quantity of soil organic carbon across a degradation gradient in Mwache Creek.

Justification of the study
Vegetated coastal ecosystems (mangroves, seagrass beds, and salt marshes) contain substantial quantities of “blue carbon” which can be released to the atmosphere when these ecosystems are degraded. For instance, mangroves contain large per-hectare carbon stocks (global stocks approximately 8 Pg C (1 Pg=1015 grams)) but due to their degradation they contribute approximately half the estimated total blue carbon emissions annually (0.24Pg carbon dioxide) (Donato et al., 2012; Pendleton et al., 2012). Indications of the capabilities of mangroves as major carbon sinks are clear, setting them apart from other coastal habitats (Donato et al., 2012). Despite their immense values, mangroves throughout the world continue to be abused, removed and degraded (FAO, 2007). Climate change impacts further threaten the existence of mangroves from the face of the earth (Gilman et al., 2008). Global loss of mangroves from 1980 to 2005 reduced mangrove area by 20% (Spalding et al., 2010). This loss has negatively affected peoples’ livelihoods, particularly communities along the coast who largely depend on mangrove products and services (IUCN, 2006).

Due to the values and the threats to mangroves, it is of interest to know the size of these carbon pools, which could lead to improvements of quantification of the global carbon stock and the sequestration capacity in different mangrove forest types. Also, in creating a baseline, carbon dynamics could determine long-term changes associated with climate change and/or land management in the mangroves (Chmura et al., 2003; Ray et al., 2011). Amid the threat of losing the ecosystem services from mangroves, an opportunity presents itself where avoiding deforestation and conservation of the carbon stocks can offer substantial benefits through climate change mitigation projects.

The present study complements previous work that aimed to determine standing biomass of mangroves of Mwache. By combining below and above ground carbon estimates, results of this study could serve as an important baseline upon which a future carbon off-set project for the area can be based along with providing an opportunity to restore the forest, ease poverty, enhance ecosystem services and also present new arguments for the conservation strategies. The study results also contribute to Kenya’s REDD readiness required to support REDD implementation by providing options for REDD+ activities. The study made use of methodologies detailed in the 2013 supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands. That way, the study may be used to inform the country’s National Inventory Submissions (NIS) to UNFCCC as well as providing country’s options regarding Nationally Appropriate Mitigation Actions (NAMAs).

Due to the absence of site and species-specific allometric equations on mangroves of Kenya, generic allometric equations developed in Asia were applied in deriving above and below ground biomass. Specific wood densities developed for the mangrove in Zambezi Delta,

Mozambique (Bosire et al., 2012) were used in the general formulae assuming similarities of the mangroves in the Western Indian Ocean region.

Scope of the study
The study was carried out in the mangrove forest of Mwache Creek. The area was chosen considering its geographical location and pressures; being a peri-urban forest where human disturbances are considerably higher compared to other remotely situated forests, and having been negatively impacted by extreme events (El nino). The forest was categorized into five sites depending on structure and location; KPA, Bonje, Mwakuzimu, Mashazani and Ngare. KPA represented islands within the creek which have resulted from accretion. KPA has young over- wash forest of Sonneratia alba. Given their location, the islands were less degraded compared to the rest of the sites. Mwakuzimu and Ngare were moderately impacted sites of mixed species stands while Mashazani and Bonje were highly impacted sites. Field sampling was done for a period of two months while laboratory analysis was carried out for a period of three months.

Limitations of the study
During the study duration, a number of limitations were encountered. There was lack of past data and a detailed vegetation map of the area that would have enabled precise temporal comparison in the biomass and carbon stock dynamics. There was also lack of an elemental analyzer for the carbon analysis which necessitated the use of a semi-quantitative method (colorimetric method) in deriving the conversion factor from organic matter to organic carbon. The absence of local factors also necessitated the adoption of specific wood densities from Mozambique, generic allometric equations from the Americas and Asia, and wood carbon concentrations from Mexico.

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